The invention concerns a gauge for measuring the thickness of an unsupported web. The gauge includes one sensor above and another sensor below the web. Each sensor includes a laser which projects its beam on the web and a detector which receives the beam reflected from the web. The laser and the detector are positioned in relation to the web such that they can carry out triangulation measurements of the distance of the web from the laser and the detector. The web can be one that is produced continuously or discontinuously, and can be a sheet of any material, for example.
A wide range of thickness measuring procedures and instruments are known. The traditional approach utilizes the absorption of ionizing radiation and allows for the requisite precision, subject to long term operation in aggressive conditions. One drawback of this technique, however, is that it employs radioactive sources, which can be detrimental to the environment and also constitutes a health hazard.
Infrared, capacitative, inductive, and laser-triangulation methods have accordingly been developed. All of these entail specific drawbacks, however, which are not discussed herein. All of these procedures and instruments depend on a sensor or sensors that travel along a respective sensor track that extends across the web perpendicular to its direction of travel. Precision depends not only on the level of resolution necessitated by the particular process but also on the straightness, true direction and stability of the tracks. As long as the webs being measured are less than 150 cm wide, the sensor can be secured to a stable C-shaped support. Measuring the thickness of webs 150 cm or wider, however, requires a rectangular frame, whose geometry dictates that it cannot be as stable as a C-shaped support. When the thickness of such a wide web is being measured, the track that the sensor travels on may sag and this will result in inaccurate measurements.
Positioning an eddy current detector in the sensor to compensate for the sag is known. The detector remotely monitors the surface of a reference cylinder while the other components of the sensor monitor the surface of the web. The thickness of the web is calculated from the difference between the two results. Since this approach employs a reference cylinder, it is inappropriate for measuring webs that arrive of the sensor unsupported.